1. Field of the Invention
The present invention relates to a steering apparatus which performs steering assist by using a driving force of an electric motor.
2. Description of the Related Art
FIG. 7 is an example of a steering apparatus provided in a vehicle.
The steering apparatus includes a cylindrical housing which is fixed to the front portion of the bottom of a vehicle body. The housing extends in a lateral direction, which is a width direction of the vehicle body, and the both ends thereof are opened.
A rack shaft is inserted into the housing so that the rack shaft is movable in the lateral direction and is not rotatable about its own axis. The left and right ends of the rack shaft are connected to left and right front wheels via tie rods, respectively. A screw groove and a rack tooth portion are formed on two parts of the outer peripheral surface of the rack shaft respectively so that the screw groove and the rack tooth portion are at positions different from each other.
Furthermore, the steering apparatus is provided with a pinion shaft, which is meshed with the rack tooth portion of the rack shaft, and a steering shaft (not shown), which is connected to the pinion shaft at the lower end thereof and is fixed to a steering wheel (not shown) at the upper end thereof.
Moreover, a rotary nut of a well-known ball screw nut is rotatably supported by the inner peripheral surface of the housing via a bearing (not shown).
In addition, an electric motor (not shown) provided in the housing is connected to the rotary nut. When the electric motor is driven, the rotary nut is rotated with respect to the housing, and a plurality of balls, which are components of the ball screw nut, circulate through a passage provided in the screw groove of the rack shaft and the rotary nut. Therefore, the rack shaft slides in the lateral direction with respect to the housing.
When a driver rotates the steering wheel, the steering shaft and the pinion shaft rotate together with steering wheel. Then, the rack shaft meshed with the pinion shaft relatively slides with respect to the housing in one of the lateral direction, and thereby steering angles of the left and right front wheels are changed.
Further, when the driver rotates the steering wheel, a steering torque generated in the steering shaft is detected by a steering torque sensor. Then, a control device (calculator) calculates a target steering assist torque corresponding to the steering torque, and operates the electric motor so as to generate the target steering assist torque. As a result, the driving force of the electric motor is transmitted to the rotary nut, thereby the rotary nut rotates. In this case a direction of rotation of the rotary nut is a direction to slide the rack shaft in the one of the lateral direction. In other words, rotative force (torque) of the nut becomes a force to assist the sliding motion of the rack shaft along with the steering operation.
By the way, grease for operating the ball screw nut smoothly is applied to the ball screw nut.
If water entering the housing from the outside of the housing comes into contact with the grease, the grease applied to the ball screw nut is washed away by the water. If the grease is washed away, motion of the ball screw nut becomes unsmooth, and/or durability of the ball screw nut decreases.
Therefore, in recent years, there is a need to prevent water from washing away grease applied to the ball screw nut by a seal member.
Therefore, in the steering apparatus of FIG. 7, a first seal member and a second seal member, both of which are constituted by elastic material and have an annular shape coaxial with the rack shaft and the housing, are fixed to two portions of the inner peripheral surface of the housing, respectively. The first seal member is in sliding contact with a first outer peripheral portion, which is a cylindrical surface located between the screw groove and the rack tooth portion of the rack shaft, in a water tight manner. On the other hand, the second seal member is in sliding contact with a second outer peripheral portion, which is a cylindrical surface located on a side opposite to the rack tooth portion with respect to the screw groove of the rack shaft, in a water tight manner.
When a torque of the pinion shaft and/or a torque of the electric motor are transmitted to the rack shaft so that the rack shaft moves in the lateral direction with respect to the housing, the first seal member and the second seal member are in sliding contact with the first outer peripheral portion and the second outer peripheral portion of the rack shaft, respectively. Regardless of relative position of the rack shaft with respect to the housing in the lateral direction, the first seal member and the second seal member are in contact with the first outer peripheral portion and the second outer peripheral portion of the rack shaft, respectively.
Therefore, even when water enters the housing through both opening ends of the housing, there is a low possibility that the water passes through a space between seal members (the first seal member and the second seal member) and the outer peripheral surface of the rack shaft, and the water washes away grease applied to the ball screw nut.
It should be noted that Japanese Unexamined Patent Application Publication No. 2006-256528 discloses a background technology related to the present invention.